An AMLCD may, for example, be a transmissive display that is illuminated by a backlight placed on the opposite side of the display to an observer. An AMLCD may alternatively be a transflective display which may be illuminated by a backlight in low ambient lighting conditions or by reflected ambient light in bright ambient lighting conditions. It is desirable to control the intensity of the backlight in dependence on the ambient lighting conditions, so that an image displayed on the AMLCD is always clearly visible to an observer but is not uncomfortably bright. A further consideration is that, particularly in the case of an AMLCD incorporated in a mobile device, such as a mobile telephone, it is highly desirable to reduce the power consumption of the backlight so as to maximise battery life. Accordingly, in the case of a transflective display, the backlight is preferably operated at a low intensity in very low ambient lighting conditions, operated at a higher intensity in medium ambient lighting conditions to ensure that an image remained visible to an observer, and switched off in ambient lighting conditions that are bright enough to provide a displayed image using only reflected ambient light.
It is therefore known to provide a mobile AMLCD device with an Ambient Light Sensor (ALS) system, and to control the power level of the backlight in dependence on the output of the ALS system. In order to be able to detect the full range of ambient lighting conditions from bright sunlight to near darkness, such an ALS system requires a high dynamic range and this necessitate detection of low light levels across a wide operating temperature range. Typically, an ALS system is required to be sensitive over a wide range of incident light levels and the typical operating temperature range of a mobile LCD device.
It would be advantageous from both cost and mechanical points-of-view to monolithically integrate all components of an ALS system, including the photo-detection element itself, onto the TFT (thin film transistor) substrate of the display. However, is difficult to achieve the required sensitivity for an ALS system, particularly at low light levels, with an ALS system that is monolithically integrated onto a display substrate. This is due to the relatively poor performance of photodetection elements fabricated in polysilicon processes, such as that used in the fabrication of a typical display TFT substrate.
Conventional ambient light sensor systems may employ discrete photodetection elements or they may employ photodetection elements integrated onto a display substrate. In the case of discrete photodetection elements, the process technology for manufacturing the element is optimised for maximising the sensitivity of the device. In the case of integrated photodetection elements, such as a photodetection element integrated on a CMOS Integrated Circuit (IC), the processing technology is however a compromise between maximising the sensitivity of the photodetection element and maximising the performance of the peripheral circuitry.
In the case of an AMLCD with a monolithically integrated ambient light sensor, the basic photodetection device used must be compatible with the fabrication processes used in the manufacture of the display TFT substrate. A well-known photodetection device compatible with the standard TFT process is the lateral, thin-film, polysilicon p-i-n diode, the structure of which is shown in cross-section in FIG. 1.
In the thin-film p-i-n diode of FIG. 1, a layer 3 of silicon is deposited over a substrate 1; there may be one or more intermediate layers between the layer of silicon and the substrate 1, and one such intermediate layer 2 is shown in FIG. 1. The silicon layer 3 is deposited as an intrinsic (i.e., not intentionally doped) layer, and subsequently one end region 3a of the silicon layer 3 is doped p-type and the other end region 3b of the silicon layer 3 is doped n-type. The remaining part of the silicon layer remains as a region 3c of intrinsic silicon. A gate insulating layer 4 and a dielectric layer 5 are grown over the substrate 1 and the silicon layer 3, and via holes are then formed in the gate insulating layer 4 and dielectric layer 5 so as to expose the p-type and n-type doped regions 3a,3b of the silicon layer. Finally, electrodes 6, 7 are provided so as to contact the p-type and n-type doped regions 3a,3b of the silicon layer respectively.
FIG. 2 is a schematic illustration of the p-i-n diode of FIG. 1 in use. The detailed operation of this device is somewhat complicated but, in brief, when the bias VA maintained across the electrodes 6,7 is negative as referenced (as shown in FIG. 2 where Vanode is a negative voltage applied to the anode and the cathode of the device is grounded), two photosensitive depletion regions A,B are formed, one at the interface between the p-type silicon region 3a and the intrinsic region 3c and the other at the interface between the n-type silicon region 3b and the intrinsic region 3c. When the device is illuminated some of the incident photons are absorbed in the semiconductor material via the photoelectric effect, with each photon that is absorbed creating (at least) one electron-hole pair. Such photo-generated electron-hole pairs, known as carriers, will be created throughout the illuminated volume of the device. In general however it is only those carriers that are created either within a depletion region, or else sufficiently close to a depletion region to be able to diffuse into it, that are able to contribute to the photo-generated current and therefore be detected by the device. In FIG. 2 only the electrons from depletion region A and the holes from depletion region B can contribute to the photocurrent since the carrier lifetime in the undepleted intrinsic region 3c at the centre of the device is short and thus carriers swept under the influence of the depletion region electric field into the undepleted intrinsic region 3c will almost inevitably recombine.
One problem in providing an integrated ALS system is that photodiodes fabricated in a polysilicon TFT process have a much lower sensitivity than photodiodes fabricated in bulk technologies (such as CMOS), for two principal reasons:
Firstly the volume of semiconductor material that is photosensitive (the device's depletion region) is generally quite small. In particular the depth of the thin film layer of material is typically designed to be only a few tens of manometers, and as a result a large fraction of the illuminating radiation passes straight through the device unabsorbed and therefore undetected.
Secondly the dark current generated by thin film devices tends to be higher than in bulk devices. The dark current, defined as the diode leakage current under the condition of no illumination, is highly dependent both on temperature and the electric field across the device. It is therefore also extremely sensitive to the potential difference applied to the photodiode anode and cathode terminals.
Accordingly, an ambient light sensor comprising a thin-film polysilicon photodiode is likely to exhibit poor sensitivity and low dynamic range.
EP 1 394 859 describes lateral photodiode fabrication in a p-Si TFT process, with a novel processing technique used to suppress the leakage current. The implementation of this device in a circuit arrangement will still however operate the lateral photodiode in a mode where the lateral electric field is quite high. The leakage current will therefore still be relatively high. Therefore, whilst the processing technique described may be effective in reducing the leakage current, the implementation described is unlikely to increase the sensitivity of the photodiode sufficiently for use in a high dynamic range ALS system.
S. V. Karnik at al. describe, in “Novel multiple lateral Polysilicon p+-n-n+ and p+-p-n+ Diodes”, Proceedings of SPIE Vol. 4295 (p. 120-124, Flat Panel Display Technology and Display Metrology II), 2001, the use of poly-silicon thin film diodes connected in series as electrical circuit elements in forward and reverse biased modes of operation. The principal advantage of such an arrangement is that the leakage current in reverse biased mode of the series connected arrangement is reduced. No mention is made however of the use of these structures as photodiodes
As well as the photodetection element described above, a practical ambient light sensor system in an AMLCD will also contain:
(a) bias circuitry to control the photodetection element and sense the photo-generated charge; and
(b) output circuitry to supply an output signal (analogue or digital) representing the measured ambient light level.
The display will further contain (c) a means of adjusting the display operation based on the measured ambient light level, for example by controlling the backlight intensity.
Many suitable types of output circuitry (i.e. amplifiers or analogue-to-digital converters for analogue or digital output respectively) are well-known. Similarly, the concept of dynamically adjusting a backlight intensity based on the measured ambient light level is known as disclosed, for example, in WO2005076253A1 or by K. Maeda et al. in “The System LCD with Monolithic Ambient-Light Sensor System”, Proceeding of SID 2005, Vol. XXXVI, Book 1, p 356.
Limitations of current ALS systems, particularly monolithically integrated systems, may lie not only with the photodetection device (as described above) but also with the level of performance of the bias circuitry used to control their operation. The bias circuitry should be arranged to robustly control the operation of the photodetection device such that, ideally, it maximises the sensitivity of the photodetection device and allows a consistent photodetection device current to be measured free from the effects of process variations in manufacture of components of the photodetection device and/or bias circuitry, electronic interference and temperature effects.
US 2005/134715 describes an active pixel sensor comprising of a photodiode plus integrator circuit whereby feedback is used to optimally bias the photodiode so as to maximise the sensitivity of the photodiode. A disadvantage of this method is the requirement to interrupt the illumination supply to the photodiode in order for the feedback mechanism to adjust the operating bias across the photodiode. Additionally the circuit embodiments described would be difficult to replicate in polysilicon to the required degree of precision.
Maeda et al. (above) describe an LCD that incorporates an ambient light sensor comprising a lateral p-i-n photodiode and analogue processing circuits that are integrated directly onto the display substrate. The bias circuit disclosed in this document applies a high potential across the photodiode terminals thus generating significant photodiode dark current and limiting the sensitivity of the system.
EP 1128170 and US 2005/0205759 each describe a method whereby the current through a photodiode is measured and compared with a reference value. The photodiode bias circuit is then adjusted according to whether the measured current is higher or lower than this pre-determined reference value. EP 1128170 describes how the photodiode bias can be adjusted over a relatively wide range to cope with large changes in the incident light level. Thus by choice of a suitable reference value the photodiode can be operated in its most sensitive region at low incident light levels, but for higher incident light levels the bias voltage may be changed so as to avoid saturation of the output signal. US 2005/0205759 particularly refers to an optical receiver in a communications system and describes how the photodiode bias voltage can be dynamically controlled so as to optimise the value of any given detection performance parameter.
US 2003/0122533 similarly describes a circuit to control the bias applied across a photodiode based on a measurement of the generated current. In this case the biasing circuitry described fulfils a requirement to vary the applied bias over a large range. The method employed for determining the bias to be set is similar to EP 1128170 and US 2005/0205759, and is based on detection of the photodiode current and the use of a feedback mechanism.
The above methods of biasing would not in general be applicable to an ALS utilising integrated thin film photodiodes. Additionally it is unclear whether a feedback mechanism involving detection of the photodiode current could facilitate sufficiently precise photodiode biasing to meet the sensitivity requirements of an ALS system using integrated thin film photodiodes.
GB 1 175 517 discloses a photosensor formed of a plurality of series-connected “photo-sensitive bodies”. In operation a bias voltage is applied across the combination of the photosensor and a load resistance.
U.S. Pat. No. 5,117,099 describes a scheme whereby the currents from the detection and reference photodiodes are subtracted in the current domain. This is achieved by arranging the detection and reference photodiodes in a loop, with the anode of the detection photodiode connected to the cathode of the reference photodiode, and the cathode of the detection photodiode connected to the anode of the reference photodiode. This provides compensation for variations in ambient temperature and stray light.